In harmonically mode-locked lasers, an amplitude modulator may be used to generate a train of pulses at the round-trip rate of the laser, or a multiple, N, thereof. The repetition rate of the laser cavity is given by fo=N(vg)/P, where vg is the group velocity of the pulse in the propagating medium. P equals the perimeter of the cavity in the case of a ring laser or equals 2L in the case of a linear cavity, where 2L is the round-trip length of the cavity. This technique, called active mode-locking, requires an essentially perfect synchronization (and phase control) of the electrical signal to the modulator with the optical pulse circulating in the cavity.